Alcohol use remains common among persons living with HIV (PLWH), creating a significant barrier for mitigating transmission, achieving viral suppression, and reducing comorbidities. Current alcohol interventions are not widely accessed by PLWH and often have limited long-term effects partly due to limited self- management and continuing care outside of clinical settings. The recent development of wearable biosensors provides an unprecedented opportunity for improving alcohol interventions through continuous monitoring and individualized feedback. Currently, the Secure Continuous Remote Alcohol Monitor (SCRAM) ankle bracelet is the only validated transdermal alcohol monitor available on market. It is mainly designed for law enforcement application, and its research application is limited due to its large size, high cost, and social stigma (perceived as house arrested). The newest generation of wrist-worn transdermal alcohol biosensor (BACtrack Skyn) is designed for consumer use. It is light, less expensive, shaped like a fitness wristband, and uses a smartphone app for real-time display. However, this new wrist worn biosensor has not been validated for research use. The overarching goal of this research program is to improve alcohol intervention and eventually related clinical outcomes (e.g., liver function) among HIV+ drinkers through biosensor-assisted intervention. Research shows HIV infection can influence alcohol metabolism (i.e., higher blood alcohol levels), so it's necessary to validate the new alcohol biosensor in HIV+ drinkers in addition to the general population. The primary goal of this project, as a step toward the development and evaluation of a biosensor-assisted alcohol intervention, is to validate the newly available wrist worn transdermal alcohol biosensor (BACtrack Skyn). We propose to recruit 20 HIV+ and 20 HIV- drinkers to validate the Skyn biosensor in both laboratory and real life settings. The two specific aims are: (1) To assess validity and reliability of the Skyn biosensor through comparison with laboratory grade breathalyzer in two fixed-dose lab alcohol administration sessions, and (2) To validate Skyn biosensor in daily life using a 2-week ecological momentary assessment (EMA), and evaluate its usability and acceptability for daily alcohol monitoring. We will also explore factors (e.g., gender, HIV status) that may be associated with individual differences in biosensor readings (e.g., peak, time-to-peak). This project will be the first study to validate the Skyn wrist biosensor and evaluate its utility for research and clinical applications. If the findings confirm the validity, reliability, and usability of the Skyn device, it may fundamentally improve the alcohol and HIV research field with a more reliable, valid, and user-friendly tool for alcohol monitoring and intervention. This study may also provide information on user desired features in a wrist alcohol biosensor and its app to be incorporated in future intervention design. This work will lay the foundation for future projects to develop and evaluate the efficacy of a biosensor-assisted alcohol intervention for HIV+ drinkers to reduce alcohol consumption and eventually improve related clinical and health outcomes.